The subject matter disclosed herein relates to a neutral point clamped (NPC) power converter, and more specifically to a fault protection system for a NPC power converter.
Power converters are employed in many different power conversion applications such as, for example, variable speed drives, wind and solar converters, power supplies, uninterruptable power supplies (UPS), and static synchronous compensators (STATCOM). The power converter is connected to a power grid through a line inductor, a circuit breaker, and a step-up transformer. In one type of power converter, a three-level neutral point clamped (NPC) topology is used. An NPC converter typically includes three phase legs and two series connected DC bus capacitors. Each phase leg includes four series connected switches, where each switch has a parallel freewheeling diode. Two additional diodes, which are referred to as NPC diodes, are connected between the leg and a DC bus mid-point.
In the event one of the NPC diodes is inoperable, a short circuit condition may be created. The charging current, which is also referred to as the fault current, flows through one of the DC bus capacitors as well as through a number of the freewheeling diodes. The charging current will charge the capacitor and increase the voltage to a level that is significantly above the nominal conditions of the parallel freewheeling diodes and the DC bus capacitor. For example, the DC bus capacitor may have a nominal voltage of 1150 Volts, however the charging current may reach levels as high as about 1900 Volts. This may cause the power converter to become non-operational.
In one approach, a crowbar circuit may be employed to protect the power converter in the event one of the NPC diodes is inoperable. The crowbar generally prevents current from flowing to a DC bus of the converter in the event a fault is detected. However, a crowbar circuit adds cost and complexity to the existing power converter as well.